27 CFR 479.81: Tax scope.

Except as otherwise provided in this part, each NFA firearm transfer in the United States is subject to a transfer tax. The Director indicates that the transferor has paid the tax and ATF has approved the application by adding a stamp of the proper denomination, bearing the words “National Firearms Act.” The Director maintains the stamps and must affix or apply one to each approved application form, as provided in this subpart. The stamps may be adhesive, in electronic form, or in another form designated by the Director.